The present invention relates to optical communications systems and more particularly to amplification in optical communications systems.
The explosion of communications services, ranging from video teleconferencing to electronic commerce, has spawned a new era of personal and business interactions. As evident in the rapid growth of Internet traffic, consumers and businesses have embraced broadband services, viewing them as a necessity. However, this enormous growth in traffic challenges the telecommunication industry to develop technology that will greatly expand the bandwidth limitations of existing communications systems. Further improvements in optical communications hold great promise to meet the demands for greater and greater bandwidth.
Wavelength Divisional Multiplexing (WDM) technology permits the concurrent transmission of multiple channels over a common optical fiber, thus expanding available bandwidth and providing other advantages in implementation. When it is necessary to recover data from the WDM signal, the individual wavelength components are isolated from one another and converted to electrical form by optical receivers. These optical receivers only operate correctly when the power level of their inputs is within a specified dynamic range. Typically amplification must be provided to bring the power level of the optical signals up to the necessary level due to losses in transmission and elsewhere.
FIG. 1 depicts a prior art approach to amplification within a WDM receiver system 100. WDM receiver system 100 has as its input a composite WDM signal 102 that includes, e.g., up to 200 wavelength components located on WDM channels spaced 25 GHz apart. An optical wavelength router (OWR) 104 incorporates a first deinterleaving block 106 that divides the 25 GHz grid into two grids having 100 WDM channels at 50 GHz spacings. Deinterleaving blocks 108 and 110 then further divide these two grids into four grids of 50 channels each at 100 GHz spacings. Each such grid is equipped with an amplifier 112 to bring the signal power level up to the level needed for correct optical receiver operation. A set of demultiplexers 114 then complete the separation of the WDM signal into its individual wavelength components.
Due to various wavelength-selective effects in the WDM link and demultiplexing components, there must be a way of varying gain across the overall grid. Otherwise, certain groups of WDM channels will have power levels outside the required dynamic range. This is particularly true when the dynamic range is relatively narrow as is the case with high data rate systems where the individual wavelength components are each modulated by 10 Gbps data streams or even higher data rate streams. Furthermore, these wavelength-selective effects are dependent on the particular installation and will vary over time. Unfortunately, each of amplifiers 112 may provide flat gain across the entire spectrum occupied by the 200 channel grid with no provision for adaptive equalization.
One way to vary gain across the spectrum to assure optimal receiver performance would be to install a variable gain optical amplifier with its own pump for each channel following demultiplexers 114. By controlling the pump powers of the individual pumps, an optical equalization function may be performed. Alternatively, a variable optical attenuator (VOA) may be installed for each channel. Unfortunately, these approaches are both very expensive and space-inefficient. Their expense and cumbersomeness increase further as the number of channels increases.
What is needed is an amplification architecture for WDM receiver systems that provides an appropriate amount of amplification for each WDM channel while economizing on component cost and space consumption.